Nintendo announced via Twitter that Mario Kart Tour will be delayed until "summer of 2019" to "improve the quality of the application." The Mario Kart spinoff was originally planned to land on both the Apple App Store and Google Play in March of 2019.
Mario Kart Tour will be the fifth mobile game released by Nintendo since the video game company announced its partnership with DeNA in 2015. Little is known about the new Mario Kart title, but it is safe to assume it will not be a freemium unlock like Super Mario Run. Nintendo revealed in a 2017 earnings call that Fire Emblem out earned Mario Run with less than 10% of the downloads.
Super Mario Run is free to download with the option to unlock the full game for the one time fee of $9.99. All of Nintendo's App Store releases since Super Mario Run have been free to download with in-app purchases. This includes Animal Crossing: Pocket Camp, Dragalia Lost and Fire Emblem Heroes, all of which have earned more money than Mario Run.
This doesn't have to be bad news for those who dislike the free-to-play business model, as it is possible to make free game that are still fun to play. We will just have to wait and see as Nintendo releases more details about Mario Kart Tour in the future. The game is expected to be detailed in a dedicated Nintendo Direct closer to its release this summer.